98 GEOLOGICAL SURVEY 
fora few miles around the country, but present nothing that need be described here. Among the 
principal are Miller’s, Huff’s, and Belk’s. As might be expected, the streams around the mines con- 
tain gold, and the accumulations of loose materials in their beds are washed accordingly. 
CHAPTER IV. 
Disintegration and denudation of the Primary and Metamorphic Rocks——Paleozoic Rocks.— 
New Red Sandstone. 
There is scarcely any thing more striking in the face of the country, in this part of the State, than 
the great extent and depth to which the disintegration of the rocks has proceeded. Were it not 
‘for the occasional blocks of granite that lie scattered here and there, a person whose observations 
were confined to the surface, would scarcely suppose himself travelling over the upturned edges 
of a series of rocks. It is not difficult to understand how the surface of rocks, long exposed 
to the action of atmospheric changes and agencies, should every where present evidences of waste 
and decay, and that these should be in proportion as the materials of which the rocks are com- 
posed, are subject to decomposition by such causes. On examining the weathered surface of a mass 
of granite, it will generally be seen that the feldspar is depressed or worn away, while the quartz 
stands out in relief; and if hornblende be present this result will be still more striking. This is 
simply the consequence of the decomposition of those minerals. Feldspar is composed of silica, 66 
parts ; alumina, 18 parts; potash, 14 parts; and lime, 2 parts. Rain-water contains carbonic acid, 
and will therefore decompose feldspar by the combination of the carbonic acid with its potash.— 
The salt thus produced is highly soluble, and is washed away by every shower, leaving behind a 
white powder, which is the silica and alumina of the feldspar, and constitutes the porcelain clay, 
which is always found where feldspathic granite is disintegrating. The grains of quartz of the 
granite being indestructible by such agencies, remain projecting from the surface. 
When hornblende is present, decomposition is produced by the oxidation of the iron and by the 
combination of carbonic acid with the lime and magnesia of that mineral; for the composition 
of hornblende is silica, 46; magnesia, 16; lime, 14; alumina, 12; and iron, 14. 
The first appearance of disintegration of granitic rocks is presented by the loss of the peculiar 
lustre of the feldspar—a change which takes place long before any signs of crumbling or disinte- 
gration can be perceived: a fact that may serve as a guide in the selection of granite for building 
purposes. 
This opaque or milky appearance of the feldspar is often seen extending for several feet into the 
rock, while the latter yet remains quite solid. At Horse Creek and other localities along the boun- 
dary of the Tertiary, instances of this sort are common. Generally, however, when decomposition 
